Thrombin belongs to the group of serine proteases and plays a central part in the blood coagulation cascade as terminal enzyme. Both the intrinsic and the extrinsic coagulation cascade lead, via a plurality of amplifying stages, to the production of thrombin from prothrombin. Thrombin-catalyzed cleavage of fibrinogen to fibrin then initiates blood coagulation and aggregation of platelets that, in turn, due to the binding of platelet factor 3 and coagulation factor XIII and a large number of highly active mediators, enhance thrombin formation.
To elaborate, the mechanism of blood coagulation normally occurs in a cascade of two possible routes. One of the routes, the extrinsic blood coagulation, starts with the liberation of thromboplastin and the activation of factor VII. Activated factor VII (i.e. factor VIIa) in turn activates factor X, followed by an activation of factor V and factor II (prothrombin). Factor IIa (thrombin) converts fibrinogen into fibrin at the end of the cascade.
The other route, the intrinsic blood coagulation, occurs via an activation of factor XII by contact with and subsequent activation of factor XI, factor IX and factor X in the presence of calcium and factor VIII, followed by an activation of thrombin (factor II) to activated thrombin (factor IIa) which triggers coagulation by cleaving fibrinogen to fibrin. Thus, activated thrombin (factor IIa) plays a role in both routes of the blood coagulation cascade.
Hitherto, there has been an intensive search for anticoagulants that can particularly be utilized in the treatment of cardiovascular disease, e.g. septic shock, thromboses, embolisms, atherosclerosis and cardiac infarctions, furthermore in case of blood transfusions or following surgery. One method of suppressing the coagulation of blood is the direct administration of substances that modulate thrombin or other coagulation factors. The identification of such substances thus represents a long-felt and ongoing need in the art.
E2F refers to a family of transcription factors (also referred to herein as the “E2F family”, which includes but is not limited to E2F-1, E2F-2, E2F-3, E2-F4, E2-F5 and E2-F6), and E2F activity is plays a role in a wide variety of proliferative events. E2F activity is controlled as the end result of G1 cyclin dependent kinase regulatory cascades that involve the Rb family of proteins. See, e.g., Sherr, C. J., Cell, 73:1059-1065 (1993); Hunter, T., Cell 75:839-841 (1993); Nevins J. R., Science, 258:424-429 (1992); Helin, K. and Harlow, E., Trends Cell Biol. 3:43-46 (1993); La Thangue, N. B., Trends Biochem. Sci. 19:180-114 (1994); Sherr, C. J.; Roberts, J. M., Genes Dev. 9:1149-1143 (1995); Weinberg, R. A. Cell 81:323-330 (1995); Harbour, J. W. and Dean, D. C., Genes and Development 14:2393-2409 (2000); and Black, A. R. and Azizkhan-Clifford, J., Gene 237:281-302 (1999). Thus, ligands that selectively bind to an E2F family member would play a role in the control of cell proliferation, which is of central importance to the proper development of a multi-cellular organism, the homeostatic maintenance of tissues, and the ability of certain cell types to respond appropriately to environmental cues.
Tie2 is an endothelial receptor tyrosine kinase (RTK) that is required for both embryonic vascular development and pathological angiogenesis. Tie2 is unique among RTKs in that it has two ligands with apparently opposing actions. Angiopoietin-1 (Ang1) is an activating ligand while Angiopoietin-2 (Ang2) is thought to be a naturally occurring antagonist for Tie2. Mice lacking Tie2 or Ang1 die midway through gestation due to abnormalities of vascular morphogenesis characterized by deficient recruitment of supporting smooth muscle cells and pericytes. Moreover, Ang1 promotes endothelial cell survival and blocks the increases in vascular permeability induced by vascular endothelial growth factor (VEGF), supporting a role for Ang1 in the stabilization and maintenance of the adult vasculature. In contrast, Ang2 is required for VEGF-mediated angiogenesis, and in the absence of endothelial mitogens Ang2 may induce vascular regression.
The exact mechanism of action of the Angiopoietins remains to be elucidated. For example, high-dose Ang2 can induce downstream activation of Akt and endothelial cell survival, suggesting that it does not simply exert a dominant negative effect on Tie2. The need for improved understanding of these ligands' function is particularly important in the study of tumor angiogenesis, as several studies have now shown that inhibition of Tie2 with a soluble receptor blocks tumor growth, angiogenesis, and metastasis. However, it is unclear whether these effects are due to inhibition of the effects of Ang1 or Ang2, since a soluble receptor would bind both ligands. Specific inhibitors of these ligands have the potential to more precisely modulate Tie2 signaling and serve as valuable therapeutic agents.
Aptamers can comprise single-stranded or double-stranded nucleic acids that are capable of binding proteins or other small molecules. Aptamers that have therapeutic value would most likely bind proteins involved in regulatory cascades. The presence of the aptamer would act as a sink for the protein factors, preventing the factors from carrying out their normal functions. To date, only a few aptamers are known.
It would be desirable to identify novel aptamers that bind to factors in the coagulation cascade, to an E2F family member, or to an angiogenesis factor. Indeed, among other applications, such aptamers have utility in the modulation of coagulation, cell proliferation or angiogenesis, and would thus meet a long-felt and continuing need in the art.